1. Field of the Invention
The present invention relates to a wideband transformer/balun and more particularly, to a wideband 1:2 transformer/balun having a 30-3000 MHz bandwidth with good impedance matching. Such transformers find application in wideband, push-pull amplifiers for modern communication systems.
2. Description of the Prior Art
This invention generally relates to transformer/baluns surface mounted on printed circuit boards (PCB) and more particularly to a topology in which multiple transformers including autotransformers and transmission line transformers are wound on a single, binocular core.
An autotransformer has only a single winding, which is tapped at some point along the winding. AC or pulsed voltage is applied across a portion of the winding, and a higher (or lower) voltage is produced across another portion of the same winding. The higher voltage will be connected to the ends of the winding, and the lower voltage from one end to a tap. For example, a transformer with a tap at the center of the winding can be used with 230 V across the entire winding, and 115 volts between one end and the tap. It can be connected to a 230 V supply to drive 115 V equipment, or reversed to drive 230 V equipment from 115 V. Since the current in the windings is lower, the transformer is smaller, lighter cheaper and more efficient. For voltage ratios not exceeding about 3:1, an autotransformer is cheaper, lighter, smaller and more efficient than an isolating (two-winding) transformer of the same rating.
For radio frequency use, transformers are sometimes made from configurations of transmission line, sometimes bifilar or coaxial cable, wound around ferrite or other types of core. This style of transformer gives an extremely wide bandwidth but only a limited number of ratios (such as 1:9, 1:4 or 1:2) can be achieved with this technique.
Typical of the commercially available, PCB surface-mounted transformers are those available from Scientific Components Corporation (the assignee hereof), Model No TC2-1TG2+, Model No. TX-2-5-1 and Model No. ED-14406; from Synergy Microwave, Part N TM-2-1; and, from M/A Com, Part No MABA-007681-CT2010 and Part No MABA-010247-2R1250.
The bandwidths of these items are respectively 3 to 300 MHz, 30 to 1100 MHz, 10 to 2500 MHz, 1 to 600 MHz, 5 to 1200 MHz, and 1 to 1600 MHz which ranges do not approach those of the present invention.
Further, while the topology of the M/A Com devices is unknown, none of the other items share the topology of the present invention.
U.S. Pat. No. 6,963,255 entitled POWER SPLITTER teaches a binocular ferrite core transformer mounted on a modular substrate. A first winding is composed of three wires wound around a first leg of the binocular core and a second winding is composed of two wires wound around a third leg of the binocular core. The ferrite binocular core is glued to a low temperature cofired ceramic (LTCC) substrate, containing passive components as well as interconnect vias, and the substrate top surface has conductive pads that the wires are soldered to. The circuit formed by the two windings comprises a matching transformer and a divider. The graphs for this transformer circuit show performance extending to only about 1,000 MHz, and the substrate is a complex and expensive component to manufacture.
U.S. Pat. No. 7,012,485 entitled MINIATURE WIDEBAND RF CHOKE teaches a binocular ferrite core glued to an LTCC substrate and having a single wire wound in specific numbers of turns around each of the three core legs. The LTCC substrate contains passive components as well as interconnect vias, and the substrate top surface has conductive pads that the wires are soldered to. While the performance of this circuit extends out to at least 10,000 MHz, the circuit has only a single winding and only functions as a wideband choke.
A more capable transformer circuit would be a dual output autotransformer functioning as a transformer/balun with good impedance matching over a frequency range of 30 MHz to 3,000 MHz while having a highly miniaturized form factor, be fabricated out of readily available materials, and be surface mountable using standard robotic pick and place machinery.
The present invention has the same footprint as the Model TC2-1TG2+ and the Synergy TM-2-1; however both of these transformers operate over much lower frequency ranges.